There have recently appeared many electronic devices such as cell phones and mobile computers, for which attempts have been made for size- and weight-reduction. With such a trend, there have been intense attempts to develop a battery, particularly a secondary battery as a portable power source for an electronic device. A lithium ion secondary battery, inter alia, has gathered attention with an expectation that it could achieve a high energy density, and many studies have been made for a thin and foldable, that is, flexible battery.
A lithium ion secondary battery generally has a cathode, an anode and a separator intervening between a cathode and an anode, where the separator is impregnated with an electrolyte solution as an electrolyte. Because an electrolyte solution is a liquid, a lithium ion secondary battery involving an electrolyte solution exhibits excellent ion conductivity, that is, excellent battery performance, but requires a firm casing for enclosing an electrolyte solution and avoiding an accident by impact in order to prevent leakage of liquid or a fire due to short circuit, which imposes a limitation on the shape of a battery, leading to difficulty in making a battery thin and lightweight.
In contrast, there is known a lithium ion secondary battery involving a totally solid state polymer electrolyte (hereinafter, referred to as “all solid polymer electrolyte”) in which an electrolyte salt is dissolved in a polymer compound. A lithium ion secondary battery involving an all solid polymer electrolyte does not require a special structure for preventing leak. Furthermore, an all solid polymer electrolyte can be adhered to an electrode, a separator and/or the like, so that battery strength and shape preservation can be improved. It is, therefore, very effective for making a battery thinner and improving battery flexibility. It is also effective for providing a large area battery. However, there is a problem that an ion conductivity of an all solid polymer electrolyte is substantially lower than that of an electrolyte solution.
Meanwhile, there is known a lithium ion secondary battery involving a gelled polymer electrolyte (hereinafter, referred to as “gel polymer electrolyte”) in which a polymer compound retains an electrolyte solution. A lithium ion secondary battery involving a gel polymer electrolyte has gathered attention in that because a polymer retains an electrolyte solution in a gel polymer electrolyte, adhesiveness to an active material and an ion conductivity are more excellent in comparison with a lithium ion secondary battery involving an all solid polymer electrolyte and that leakage of liquid is less likely even compared with a lithium ion secondary battery in which without a gel polymer electrolyte, a separator is impregnated with an electrolyte solution. In general, the larger a proportion of an electrolyte solution in a gel polymer electrolyte is, that is, the larger electrolyte retaining ability is, the larger an ion conductivity is.
A variety of substances have been investigated as a polymer compound for a gel polymer electrolyte, including methyl methacrylate polymers, acrylonitrile polymers and copolymers of polyvinylidene fluoride or vinylidene fluoride with hexafluoropropylene, besides ether polymers.
An ether polymer such as polyethylene oxide, polypropylene oxide and a derivative or copolymer thereof contains highly basic ether oxygens which are capable of trapping a lithium ion and which at the same time, are continuously aligned on a polymer chain, inducing efficient hopping transfer of trapped lithium ion and improving ion conductivity. Commercially available linear polyethylene oxide and polypropylene oxide, however, have a low glass-transition point and a low melting point (about 70° C. or lower), which may lead to poor durability and shape preservation at a high temperature. Furthermore, many of these polymers are soluble in a solvent, so that some measure such as introduction of a crosslinked structure is needed for improving a melting point and solvent resistance.
As a polymer compound used for a gel polymer electrolyte, vinyl acetal polymers such as polyvinyl formal and polyvinyl butyral are known. Such a vinyl acetal polymer has oxygen atoms in its structure as described for the above ether polymer.
For example, Patent Reference Nos. 1 to 3 have described a gel polymer electrolyte containing polyvinyl acetal and an electrolyte solution. Furthermore, Patent Reference No. 4 has described investigation for increasing an electrolyte solution by adjusting the number of hydroxyl groups contained in the polyvinyl acetal through chemical modification of hydroxyl groups. Furthermore, Patent Reference Nos. 5 and 6 have described a process for manufacturing a gel polymer electrolyte with excellent charge/discharge properties and shape preservation by crosslinking the acid-modified polyvinyl acetal by current application or crosslinking the polyvinyl acetal using a crosslinking promoter. There is, however, room for further improvement of ion conductivity of the conventional gel polymer electrolyte described above and cycle properties of a secondary battery produced using the gel polymer electrolyte. There is a problem that in crosslinking the acid-modified polyvinyl acetal by current application or crosslinking the polyvinyl acetal using a crosslinking promoter as described above, preparation of materials and the crosslinking step are troublesome and tends to increase a cost.
It is known that during electrophoresis of lithium ions, anions as counter ions are, of course, also electrophoresed in a direction opposite to that of lithium ions and cause an internal resistance of a battery, leading to deterioration in charge/discharge properties. Moving ability (hereinafter, referred to as “mobility”) is different between lithium ions and the anions due to difference in interaction between these substances and an electrolyte solution and a polymer compound. Therefore, as a battery repeats discharge and charge, ion bias in the inside of the battery increases, causing concentration polarization and thus deterioration in battery performance.
A polymer compound used for a conventional gel polymer electrolyte contains a moiety capable of interacting with a lithium ion but not a moiety capable of interacting with an anion, that is, an anion has larger mobility. If an anion can be strongly drawn in a polymer, mobility of the anion can be reduced and a degree of dissociation of an electrolyte salt can be improved. The larger a degree of dissociation of an electrolyte salt is, the larger an ion concentration in a gel polymer electrolyte is and the larger ion conductivity is.
There have been, therefore, attempts to improve a transport number of a lithium ion by introducing a polar group capable of trapping an anion into a polymer matrix or introducing an anion itself into a polymer structure for constraining the anion.
For example, Patent Reference No. 7 has described an ion-conductive polymer having a borosiloxane structure and an ionic conductor therewith. Patent Reference No. 7 has described that in this ion-conductive polymer, a Lewis-acidic boron in the borosiloxane structure traps an anion and segment movement of a side-chain oligo ether bond makes a single cation movement easier, improving ion conductivity. The ionic conductor described in Patent Reference No. 7 is, however, essentially an all solid polymer electrolyte free from an electrolyte solution, so that it fails to exhibit a practically acceptable ion conductivity. Furthermore, a boron-containing compound generally has problems in terms of safety.
Non-patent Reference No. 1 has described attempts for developing a polymer having an urea group which is expected to chemically interact with an anion in an electrolyte salt and reducing anion mobility. However, it cannot strongly withdraw an anion in a polymer and an anion still has larger mobility than a lithium ion.